1. Field of the Invention
The present invention relates to an electronic part having a Sn plating layer as the outermost layer of an outer electrode.
2. Description of the Related Art
In chip type multilayer capacitors and other electronic parts, for example, an outer electrode formed outside a chip body is generally installed on, for example, a printed wiring board by soldering. A configuration having a Sn plating layer as the outermost layer of the outer electrode is employed in many cases to improve soldering property at relatively low cost. This is because Sn plating layers have the merits of very satisfactory wettability and of suppressing the occurrence of defective installment when the electronic part is installed by reflow, flow or another treatment.
When a Sn plating layer is formed in an outer electrode of the electronic part, a Ni underlayer or Ni alloy underlayer is generally formed under the Sn plating layer to improve solder temperature resistance. The Ni underlayer and Ni alloy underlayer act to prevent a thick film electrode formed on the surface of the chip body from melting and being absorbed in the solder upon soldering.
Incidentally, the Sn plating layer is known to invite the formation of whisker-like protrusions called as xe2x80x9cwhiskersxe2x80x9d. Such whiskers are formed only by leaving the Sn plating layer standing at constant temperatures, but the mechanism of its formation has not yet been clarified. If the whisker is formed in an outer electrode of an electronic part installed on a circuit board, there is a possibility of an electrical short-circuit between the electronic part and an adjacent part or a wiring pattern.
As techniques for retarding whiskers in a Sn plating layer, the following techniques have been proposed. For example, there is a technique of adding a small amount of Pb, Ag, Bi or another metal to a Sn plating layer to yield a Sn alloy plating layer, and a technique of dividing the Sn plating layer into two layers, i.e., a lower layer having a smaller crystal grain size and an upper layer having a larger crystal grain size, as disclosed in Japanese Unexamined Patent Application Publication No. 2-170996. However, quality control of a plating bath is difficult using the technique of forming a Sn alloy plating layer. The technique of dividing the Sn plating layer into two layers has the following problems. Specifically, when a Sn plating layer is formed in an outer electrode of an electronic part, a Ni underlayer or Ni alloy underlayer must be formed under the Sn plating layer to improve the solder temperature resistance, as mentioned above. When dividing a Sn plating layer into two layers, the two Sn plating layers must be formed on a Ni underlayer or Ni alloy underlayer formed on a thick film electrode. This technique therefore requires two Sn plating steps and is disadvantages in cost.
In this connection, the Ni underlayer and Ni alloy underlayer formed in order to improve the solder temperature resistance of an outer electrode of an electronic part also advantageously serves to inhibit the formation of whiskers in a Sn plating layer. Accordingly, when a Sn plating layer is formed in an outer electrode of an electronic part, the formation of such a Ni underlayer or Ni alloy underlayer under the Sn plating layer can improve the solder temperature resistance of the outer electrode and concurrently can inhibit the formation of whiskers in the Sn plating layer when the electronic part is placed at constant temperatures.
However, it has been clarified in recent years that even such an electronic part having a Ni underlayer or Ni alloy underlayer formed under the Sn plating layer of the outer electrode invites the formation of whiskers if the electronic part is left under conditions where high temperature conditions and low temperature conditions are repeated. Whiskers formed under such conditions frequently have bending shapes and have lengths of hundred and several ten micrometers, in contrast to whiskers formed at constant temperatures. The mechanism of the formation of this type of whiskers has not yet been clarified, but it is supposed that the mechanism acting in this case is quite different from that of whiskers formed at constant temperatures. This supposition is based on the difference in formation conditions and difference in shapes of whiskers. In the future, intervals or spaces between individual parts and between wiring patterns will further decrease with increasing packing densities of circuit parts, and the risk of defective short-circuits due to the whiskers as above further is expected to increase more and more.
Accordingly, an object of the present invention is to provide an electronic part which can avoid the formation of whiskers in a Sn plating layer formed in an outer electrode, even in surroundings where high temperature conditions and low temperature conditions are repeated.
The electronic part comprises: a body and an outer electrode formed on the surface of the body, the outer electrode comprising a plurality of layers and having a Sn plating layer as the outermost layer. The Sn plating layer has an average crystal grain size of about 1 xcexcm or less.
It is preferable that a Ni layer or a Ni alloy layer is formed under the Sn plating layer, and a thick film electrode may be further formed under the a Ni layer or Ni alloy layer. The body is preferably a ceramic body.
A method for producing an electronic part comprises the steps of: preparing a ceramic body; forming a thick film electrode on the surface of the ceramic body; forming a Ni layer or Ni alloy layer on the thick film electrode; and forming a Sn plating layer having an average crystal grain size of about 1 xcexcm or less on the Ni layer or Ni alloy layer through a plating bath with a pH of about 3 to 10.
As is described above, according to the present invention a Sn plating layer is has a fine average crystal grain size of about 1 xcexcm or less in an electronic part including a body and an outer electrode formed on the surface of the body and having the Sn plating layer as the outermost layer, and the formation of whiskers in the Sn plating layer can be inhibited even in surroundings where high temperature conditions and low temperature conditions are repeated. When this electronic part is installed in a circuit board, the occurrence of electric short-circuits with an adjacent part or wiring pattern can be inhibited even in surroundings where temperatures vary depending on the state of use.
For the purpose of illustrating the invention, there is shown in the drawing a form which are presently preferred, it being understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown.